1. Field of the Invention
The invention relates to a method of producing a bubble-free vitreous material, and in particular bubble-free vitreous silica, from a bubbles-containing vitreous material, and in particular vitreous silica, by subjecting the latter in the heated state to elevated gas pressure.
In the optical and lampmaking fields, glasses have long been needed which are as free of bubbles as possible. Various proposals have therefore been published in the past for solving the problem of producing bubble-free vitreous materials.
2. Discussion of the Prior Art
According to German Pat. No. 310,134 dated Jan. 2, 1922, for example, bubble-free fused quartz is produced by melting quartz under vacuum and holding the molten quartz under superatmospheric pressure until it solidifies. The superatmospheric pressure employed is such that the gas bubbles are reduced to the volume which they would have at normal temperature. It is recommended that the pressure employed not be too high as otherwise the cooled quartz would have high internal stresses that would render it unfit for optical use.
A similar method is proposed in German Pat. No. 854,073 dated Oct. 30, 1952. Small-sized rock crystal or granular precipitated silica is charged to a furnace for melting, and a hydrogen atmosphere is maintained in the furnace under a pressure ranging from 10 to 150 mm Hg until the melting temperature is reached, and also for the duration of the melting operation, which takes 10 minutes, the pressure being raised to about 1 to 2 atmospheres at the end of the melting process. Now the vitreous silica so produced contains dissolved hydrogen and cannot be used directly in the production of mercury-vapor lamps, for example, since in the operation of such lamps hydrogen would be liberated by the vitreous silica that would result in an increase in the firing potential.
With said prior-art methods, the elimination of macrobubbles, in other words, bubbles which are visible to the naked eye, is assured.